The knowledge of geometrical properties of discontinuities is of crucial importance in the rock mass characterisation process. Recent advances in photogrammetry allow for an easy digitisation procedure of rock surfaces so that digital 3D models can be used for remote site characterisation.
This paper presents a methodology to digitise tunnel rock surfaces using Structure from Motion digital photogrammetry for remote measurements of discontinuities. The proposed method is applied on a 12 m long and 4 m high tunnel section of an underground research tunnel at Aalto University in Finland, which is scanned using Canon 5Ds R DSLR camera and Canon 14 mm f/2.8 and 35 mm f/1.4 lenses. The photos are then processed in commercially available photogrammetric software – RealityCapture.
As a result, a high-resolution 3D point cloud of the tunnel wall is produced. The point cloud is used for semi-automatic measurements of fracture orientations. In addition, a digital twin of the tunnel section with photorealistic surface texture is created and implemented into virtual reality (VR) system – Virtual Underground Training Environment (VUTE) developed for training of rock mass characterisation. The VUTE system enables remote visual inspection of the rock surface and virtual measurements of the orientation of discontinuities with designated virtual tools. The semi-automatic measurements extracted from the 3D point cloud using a discontinuity extractor software are compared with measurements performed in VR as well as with manual measurements performed in the tunnel. The results demonstrate that all three mapping methods identify three major joint sets with analogous orientations.
The automatic fracture mapping method achieves the highest density of the measurements, allows repeatability, and enables other parameters to be extracted automatically, such as persistence and spacing of the discontinuities. This confirms the advantage of automatic analysis of discontinuities on 3D point clouds of tunnel rock surface digitised using photogrammetry.
The discontinuities in the rock mass are one of the most critical factor that governs its geomechanical behaviour. Because of that, the knowledge of the geometrical and mechanical properties of discontinuities is a crucial component in the design of stable underground structures (Hudson & Harrison, 2000). The orientation of discontinuities is typically mapped manually using a geological compass at rock outcrops. However, manual mapping of discontinuities has several limitations. First, the timeframe for the measurements is constrained, especially in underground tunnels due to time pressure of excavation cycles. Second, it is also limited to only the accessible tunnel faces. Most importantly, manual fracture mapping is biased due to human factors, such as previous experience or motivation of the person doing the measurements (Gaich et al. 2003).